1. Field of the Invention
The present invention relates to a method of dry etching, and more particularly, to a method of plasma etching used for manufacturing semiconductor devices.
2. Description of the Related Art
Recently, as semiconductor devices have become more highly integrated, higher performance in size precision of the patterning processes has been required in order to keep up with progress in the microminiaturization of electronic circuits. Consequently, etching of silicon material (e.g., silicon and material including silicon), typically used for gate electrodes of MOS transistors, has gradually shifted from isotropic wet etching to anisotropic dry etching. For example, reactive ion etching (RIE), electron cyclotron resonance (ECR) plasma etching, and the like are used. In such plasma etching processes, thin layers of silicon material or substrates are patterned by removing particles (atoms, molecules) from a solid surface of the material, using chemical and physical reactions between the solid surface and radicals and ions which are generated in the plasma.
Halogen/halide gas etchant such as fluorine-based etchants, Freon (chlorofluorocarbon)-based etchants, and the like have been used conventionally for silicon material plasma etching. Recently, chlorine-based etchants such as Cl.sub.2 and bromine-based etchants such as HBr have been used because the chlorine-based and bromine-based etchants produce anisotropic images which fluorine-based and Freon-based etchants do not. The volatility of reaction products produced by etching reactions between the chlorine-based and bromine-based etchants and silicon material is relatively low so that the etching process does not progress automatically, (i.e., the etching process is progressed-by an ion-assist mechanism). In addition, bond energies E.sub.bond of chlorine and bromine to silicon are lower than those of fluorine and Freon to silicon, (i.e., E.sub.si-F &gt;E.sub.si-O &gt;E.sub.si-Cl &gt;E.sub.si-Br). Thus, chlorine-based and bromine-based etchants increase the etching selectivity of silicon material layers to oxide underlayers compared with fluorine-based and Freon-based etchants.
As semiconductor devices become more highly integrated, the thickness of oxide underlayers (for example, gate insulating films made of a material such as silicon oxide under silicon material gate electrodes) also has been reduced. Accordingly, in patterning silicon material, it is important to improve the selectivity between the silicon material layers and the silicon oxide layers minimize damage to the silicon oxide layers, and improve the size precision. A new dry etching technique is required which satisfies these requirements of high selectivity, less damage, and high size precision.
(1) Conventional ECR Plasma Etching Apparatus
FIG. 1 shows an example of ECR plasma etching apparatus 200 which utilizes microwaves. As shown in FIG. 1, in the plasma etching apparatus 200, a magnetron 201 generates the microwaves of 2.45 GHz, a waveguide 202 introduces the microwaves into a vacuum chamber 204 through a window 203. Solenoid coils 205 generate a magnetic field 206 in the vacuum chamber 204. The plasma 207 is generated by cyclotron motion of electrons caused by the multiplier effect of the magnetic field 206 and the electric field of the microwaves.
The solenoid coils 205 are usually divided into at least two coils in order to increase uniformity of the plasma and controllability for the plasma. By varying independently electric current values of the respective coils so an to change position of en ECR region and gradient of the magnetic field 206, the efficiency of the microwave power and the density distribution of the plasma 207 can be changed. The ECR region is a region satisfying an ECR condition of magnetic field strength of 875 Gauss in the magnetic field 206. The electric current values of the solenoid coils 205 are detected by a power meter 208.
A directional coupler 209 detects incident and reflected microwaves traveling through the waveguide 202. Power (intensity) of the incident and reflected waves is indicated by power meters 210 and 211, respectively. A signal 221 which represents the power of the reflected wave is sent to a control circuit 212 from the power meter 211. The control Circuit 212 adjusts the microwaves and minimizes the signal 221 by Changing positions of stub pins 213 which are driven by motors, so as to minimize the reflected energy and maximize an incident power of the microwaves.
As shown in FIG. 1, a wafer 214 to be etched is placed on a wafer platen electrode 216 in a reaction chamber 215. The wafer platen electrode 216 is connected to a high frequency power supply 217 and is supplied with a high frequency bias voltage. Process gas is introduced from a gas inlet 218 into the vacuum chamber 204, is used for etching, and then is exhausted through a gas exhaust 219.
In order to improve the size precision of plasma etching as described above, it is important to obtain a plasma condition suitable for fabrication of semiconductor devices by optimizing process parameters such as gas pressure and gas flow rate. More specifically, it is important to measure and obtain a preferable value of selectivity (the ratio of etch rates) in the plasma. For example, it is required to perform an end point detection (EPD) effectively and precisely in order to measure the selectivity. An example of EPD is described in Japanese Patent Publication No. 7-75230, in which density ratios of ions and radicals are monitored by using a mass analyzer.
(2) Selectivity
Selectivity is conventionally obtained by measuring actual etch rates. For example, changes in thickness of layers before and after the etching process of sample wafers are measured using an optical measuring device so as to calculate the selectivity from the measurement results. Scanning electron microscopes (SEM) are also used to observe cross-sections of the sample wafers to determine the selectivity.
However, it is time and labor consuming to measure etch rates for various process conditions by using many sample wafers. Thus, empirical knowledge such as experimental data for other etching apparatuses and available information from academic journals and the like are utilized in order to estimate etch rates. For example, a shortage in the number of actual measurements is compensated by making predictions using empirical laws such that a lower discharge voltage in the vacuum chamber is preferable. However, when the process parameters are peculiar for one etching apparatus, it is difficult to make a prediction for another etching apparatus based on the process parameters. In addition, as the number of the process parameters used for controlling the plasma condition is increases, it becomes difficult to make a comprehensive decision for setting the process parameters.
For example, in the case where a certain condition which provides an optimum value of one process parameter is found to be a particular one, the optimum value of the process parameter should be measured again by varying the process parameter under other conditions so as to re-set a general optimum value.
As a more specific example, obtaining optimum values of process parameters (RF power and gas pressure) for improving etch rate is described below. With initial conditions of RF power 100 W and pressure 0.5 Pa, and by varying the RF power while maintaining pressure of 0.5 Pa, it is assumed that an optimum value 80 W of the RF power is obtained for the highest etch rate. After similar measurements for other pressure values, it is revealed that the pressure value of 0.5 Pa is a particular condition and the optimum value of the RF power for other pressure values is 120 W. In such a case, the first measurement for the pressure value of 0.5 Pa would be useless.
Furthermore, in the case where the selectivity is dependent upon a plurality of process parameters, for example, both pressure and RF power, in order to find which of pressure and RF power parameters the selectivity most strongly depends on, it is necessary to perform a large number of measurements using many sample wafers. Determining the dependency of the selectivity on a plurality of process parameters requires a large number of measurements that usually cannot be performed effectively.
In addition, in the case where the process parameters cannot be varied separately due to intrinsic characteristics of the etching device, it is difficult to determine an optimum value for each of the process parameters. For example, total gas flow and gas pressure are not independent, and an increase of the total gas flow increases the pressure and vice versa.
As described above, the conventional methods of measuring the selectivity and determining optimum values of the process parameters for obtaining high selectivity have many problems to be solved.
(3) Size Precision of Etching Process
Size precision of plasma etching is determined by profile variations in resist masks and those in films or substrates which are subjected to etching during the etching process. Therefore, these variations in the etching process (referenced as process etch biases) should be reduced in order to improve size precision.
Plasma etching of silicon material is usually performed using organic resist masks which are used as etching masks in photolithography processes, and using halide gas plasma au an etchant. Using organic resist masks reduces the number of fabrication steps for forming the etching masks which are required for silicon oxide masks. Thus, organic resist masks are advantageous for improving the size precision and reducing the cost by reducing the number of fabrication steps. In addition, etching residues are likely to occur in stop portions of patterns when silicon oxide films are etched to form resist masks. Etching residues are not likely to occur in forming organic resist masks. As described above, organic resist masks are easier and simpler to use than are silicon oxide masks.
However, it is known that, in the case where organic resist masks are used for silicon material etching, the selectivity of silicon material layers to underlying oxide layers (silicon oxide films) is degraded compared with the case of using silicon oxide masks. The reason is believed to be that by-products from organic resist masks affect etching reactions between oxide underlayers and etchants. Accordingly, in order to improve size precision of the etching process by taking advantage of organic resist masks, it is required to increase the selectivity of silicon material layers to oxide underlayers.
(4) Improvement of Selectivity and Size Precision
A gas mixture in which oxygen gas is added to halide gas can be used as an etchant in order to improve selectivity of silicon material layers to oxide underlayers when organic resist masks are used as described above. However, additive oxygen gas reduces etching selectivity of silicon material layers to organic resist masks. Furthermore, additive oxygen gas increases deposits on sidewalls of organic resist masks, resulting in degradation of size precision.
In the case where an amount of additive oxygen gas in minimized in order to prevent degrading size precision, selectivity of silicon material layers to oxide underlayers remains insufficient. Increasing the amount of additive oxygen gas degrades size precision. Accordingly, there is a trade-off between size precision and selectivity.
In plasma etching of silicon material using organic resist masks in a mixture of halide gas including a halogen such as bromine (Br) and additive oxygen gas, banking deposits are formed on sidewalls of the organic resist masks. The banking deposits degrade size precision as described below.
FIGS. 2A to 2C schematically show steps of plasma etching a substrate (wafer) 300. As shown in FIG. 2A, the substrate 300 includes a silicon substrate 316, a gate insulating film 315 formed on the silicon substrate 316, a silicon material layer 314 such as a polycrystalline silicon film formed on the gate insulating film 315, the silicon material layer 314 being subjected to etching, and a organic resist mask 313 having a predetermined pattern.
Desorption products (silicon halide) which are produced in plasma etching of the polycrystalline silicon film 314 react with oxygen in the plasma at solid surfaces of the substrate 300. These reactions produce dissociated halogen radicals an silicon oxides. The silicon oxides are deposited on the surface of the polycrystalline silicon film 314 other than the portion where the polycrystalline silicon film 314 is bombarded by ions. More specifically, as shown in FIG. 2B, the plasma-produced silicon oxides are deposited on sidewalls 313a of the organic resist meek 313 so as to form banking deposits 317.
The banking deposits 317 serve as an-additional mask to the organic resist mask 313 beyond the original mask pattern of the organic resist mask 313. This causes an insufficient etching of the polycrystalline silicon film 314 beneath the banking deposits 317. As a result, as shown in FIG. 2C, a pattern (profile) of the etched polycrystalline silicon film 314b is shifted from the original pattern of organic resist mask 313, so that the size precision is significantly degraded. Since the influence of banking deposits depend on the resist pattern, patterns is more significant than that of dense resist patterns. This causes line width variations in line patterns. In addition, in the case of bromine-based etchants, deposit amounts of silicon oxide are increased by reactions between silicon bromide and oxygen, since the bond energy of bromine to silicon is lower than that of oxygen.
In order to solve such problems, an amount of oxygen gas may be increased in an additive etching step which is performed after the etching step of silicon material is substantially finished and an oxide underlayer is exposed. Alternatively, oxygen gas may be introduced in the additive etching step. However, practically, deciding a timing for changing the oxygen gas flow rate is difficult. Increasing or adding oxygen gas in the additive etching step causes deviations in resultant patterns of the silicon material layer.
In order to increase selectivity of silicon material layers to resist masks, the etch rate of silicon material layers may be increased by using plasma having a density as high as 10.sup.12 /cm.sup.3. In high density plasma, the ionization ratio is increased so as to enhance dissociation of etching-produced products into components, producing radicals which etch silicon material layers. Accordingly, ion-contribution to the etching is enhanced so that the selectivity is improved.
However, high density plasma causes the following problems. Electron build-up in surfaces of the substrates is necessarily increased. Local differences in accumulated electric charge causes insulation breakdown in oxide underlayers. Notches are formed on surfaces of silicon material layers in the etching. In the case where organic resist masks are used, etching-produced by-products including carbon are recombined and polymerized in the plasma and deposited on surfaces of the substrates. In addition, since the dissociation of etching-produced silicon halide is further enhanced in the high density plasma, deposits of silicon oxide on substrates and resist masks are increased. This degrades the size precision as described above. Furthermore, controlling etch rates becomes difficult due to increasing dissociation and deposit reactions.
(5) Low Damage
Additive etching is performed after oxide underlayers are exposed, in order to remove etching residues and to reduce variations in the thickness of silicon material layers due to the etch rate variations. Thus, it is required to improve the selectivity of the silicon material layers to the oxide underlayers, to reduce damage of the oxide underlayer, to prevent oxide underlayers from being overetched to silicon substrates underlying the oxide underlayers, and to prevent the degradation and insulation breakdown of oxide underlayers due to bombardment and electron build-up from the plasma.
(6) Diagnosis and Control of Etching Apparatus and Plasma Condition
Measurements of the etch rates or thicknesses of layers or firms by optical observations or by SEM observations of cross-sections of sample wafers have been used as conventional methods for diagnosing plasma conditions. However, these methods are not only time and labor consuming as described above, but also are subject to influences of the previous fabrication steps. For example, problems in a heating process performed in the step of forming a silicon material layer cause changes in etch rates and etched shapes of the layer, even if the etching apparatus operates normally to provide optimum plasma conditions.
These changes in etch rates and etch shapes are usually considered to be a result of a malfunction of the etching apparatus. However, whether the cause of such changes is due to a malfunction of the etching apparatus or not cannot be diagnosed until the etch, rates or the film thickness are measured again using other wafers. Consequently, measurements of etch rates or film thicknesses are not good indices which exactly represent plasma conditions and operation conditions of the etching apparatus, since these measurements change depending upon not only plasma conditions but also wafer conditions.
In an actual etching process, it is important to control process parameters so as to obtain an optimum plasma condition for etching, such as a condition providing maximum selectivity. For example, in the conventional plasma etching apparatus 200, plasma condition is controlled by maximizing the input power of the microwaves. That is, the plasma etching apparatus 200 adjusts the microwaves to minimize (to zero) the power of reflected waves of the microwaves which travel in the waveguide 202.
However, the minimum (zero) power of the reflected wave does not necessarily correspond to the maximum power of the input waves. In addition, the isolation of the directional coupler 209 of the plasma etching apparatus 200 is not very sufficient. Therefore, the controlling of the microwaves in the waveguide 202 cannot necessarily realize the optimum plasma condition for etching in the vacuum chamber 204.
In the plasma etching apparatus 200, the plasma condition is also controlled by changing current values of the solenoid coils 205 so as to change the magnetic field 206. However, optimum current values of the solenoid coils 205 to realize the optimum plasma condition are varied depending on many factors such as the kinds of etch gases, gas pressures for discharge, the power of the input microwaves, kinds of silicon material to be etched, shapes and sizes of the solenoid coils 205 and reaction chamber 215, and the like. Accordingly, it is required to set electric current values of the solenoid coil 205 for each combination of factors. This makes it difficult to effectively control the plasma condition.
Japanese Laid Open Patent Publication No. 6-188221 describes a method for controlling plasma conditions by detecting emission intensity of the plasma. In this method, stub pins are adjusted so as to make the detected emission intensity maximum in order to generate an efficient plasma condition. However, since a plurality of etch gases are generally used in the conventional plasma etching apparatus 200, reactions in the plasma are so complicated that it is difficult to obtain the optimum plasma condition by maximizing an emission intensity of an active species.
As described above, conventional methods for controlling process parameters to realize the optimum plasma conditions for high selectivity and to estimate an actual selectivity from the current plasma condition in the etching apparatus have many practical problems.